An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force.

Newton's First Law of motion is called the Law of Inertia. It states:AN OBJECT MAINTAINS A CONSTANT STATE OF MOTION(VELOCITY) UNLESS IT IS ACTED UPON BY A NET FORCE.This means an object at rest stays at rest and an object moving continues to do so unless some force acts on it to cause a change.Remember that net force means the vector sum of all the forces acting on an object.Inertia is generally considered to be the measure of an object's tendency to resist changes of motion when acted upon by a net force. It is directly related to the mass of the object.

the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction

1. Newton’s First Law says that objects do not accelerate spontaneously.

2. This property of matter, which causes objects to resist acceleration, has been named “inertia”.

3. Newton’s First Law is often called the Law of Inertia.

Forces on a sailboat ◊ If a sailboat is cruising at constant velocity with the wind coming from directly behind it, what must be true about the forces acting on it? ◊ The forces acting on the boat must be canceling each other out. The boat is not sinking or leaping into the air, so evidently the vertical forces are canceling out. The vertical forces are the downward gravitational force exerted by the planet earth and an upward force from the water.The air is making a forward force on the sail, and if the boat is not accelerating horizontally

then the water's backward frictional force must be canceling it out.Contrary to Aristotle, more force is not needed in order to maintain a higher speed. Zero

total force is always needed to maintain constant velocity. Consider the following made-up numbers:

boat moving at a low, constant velocity boat moving at a high, constant velocity forward force of the wind on the sail … 10,000 N 20,000 N backward force of the water on the hull … 10000 N 20000 N total force on the boat … 0 N 0 N The faster boat still has zero total force on it. The forward force on it is greater, and the backward force smaller (more negative), but that's irrelevant because Newton's first law has to do with the total force, not the individual forces.

This example is quite analogous to the one about terminal velocity of falling objects, since there is a frictional force that increases with speed. After casting off from the dock and raising the sail, the boat will accelerate briefly, and then reach its terminal velocity, at which the water's frictional force has become as great as the wind's force on the sail.

The sum of forces acting on an object is the net force Consider a car traveling at a constant velocity. Newton's first law tells us that the net external force on the car must be equal to zero. However. In the Figure shows that many forces act on a car in motion. The vector F forward represents the forward force of the road on the tires. The vector F resistance, which acts in the opposite direction, is due partly to friction between the road surface and tires and is due partly to air resistance. The vector F gravity represents the down ward gravitational force on the car, and the vector F ground-on-car represents the upward force that the road exerts on the car.To understand how a car under the influence of so many forces can maintain a constant velocity, you must understand the distinction between external force and net external force. An external force is a single force that acts on an object as a result of the interaction between the object and its environment. All four forces in the Figure are external forces acting on the car. The net force is the vector sum of all forces acting on an object. When all external forces acting on an object are known, the net force can be found by using the methods for finding resultant vectors. The net force is equivalent to the one force that would produce the same effect on the object that all of the external forces combined would. Although four forces are acting on the car in the Figure, the car will maintain its constant velocity as long as the vector sum of these forces is equal to zero.

Imagine a basketball and a bowling ball at rest side by side on the ground. Newton's first law states that both balls remain at rest as long as no net external force acts on them. Now, imagine supplying a net force by pushing each the two are pushed with equal force, the basketball will accelerate much more than the bowling ball. The bowling ball experiences a smaller acceleration because it has more inertia than the basketball does. As the example of the bowling ball and the basketball shows, the inertia of an object is proportional to the object's mass. The greater the mass of a body, the less the body accelerates under an applied force. Similarly, a light object undergoes a larger acceleration than does a heavy object under the same force. Therefore, mass, which is a measure of the amount of matter in an also a measure of the inertia of an object.

the state in which the net force on an object is zero

Equilibrium for moving objectsMoving objects are in dynamic equilibrium, not static.Sum of forces must be = 0 N if its velocity is constant. Remember: Newton’s First Law!The “balancing a deck of cards in your hand while on a moving train,” example… cards don’t know the difference.

Equilibrium for Objects at RestObjects at rest are said to be in static equilibrium.Static objects have F net = 0 N. Even though there may be many forces acting on that object they all cancel each other out.

5 N 5 N

F net = 0 N

Newton

• Sir Isaac Newton (1642-1727)

• Mathematician and physicist

• Discovered many things:– Laws of motion– Optics– Gravity– Calculus